I've been working on some loads for a 7.62x39 Bolt Action rifle. I've found that cheap factory steel case Silver Bear 125 grain soft points are chronographing 2,555 fps average from my 20" barreled rifle.
My own handloads are averaging 2127 fps with TAC and 2000 fps with N133.

I tore down the factory steel case ammo and it has 25.5 grains of powder in it that looks similar to N133 (a short cut stick powder) but I can't figure out what powder the factory is using and how they are getting 430 fps more than I'm getting.
Just thought I'd throw that out there and see what the more experienced have to say.
Accuracy is far more important to me than velocity but I don't want to leave 20% of the velocity on the table.

Well. First off QL says you can't get there from here. Nothing gets a 125gr bullet to 2555 and stays under max pressure.

You are having trouble getting the 308 to group. If the barrel is a 310 not a 308 you would loose a touch of velocity since your pressure doesn't build as much.

I just picked up a box of .310 diameter bullets today on the way home from a Cow Elk hunt. The new numbers are based on .310 bullets.
I wonder if I need to double check my chrony?
But the average fps is based on 15 shots of my loads and 6 factory loads.
The factory loads ranged from 2538 to 2574fps.
Paul I have Quickload but admittedly I don't know my way around that program very well.
What powder does Quickload say gives the fastest velocity?
I wondered if they were pushing the velocity up higher due to the steel case over brass but that doesn't make the primer any stronger?

Let me rerun it. I had the burn rate limits on powder set to low.

A custom powder in the N120-n125 range might be what the factory load is.

Cartridge          : 7.62 x 39 (.308) Russ.
Bullet             : .310, 123, Hornady FMJ 3147
Useable Case Capaci: 31.936 grain H2O = 2.074 cm³
Cartridge O.A.L. L6: 2.190 inch = 55.63 mm
Barrel Length      : 20.0 inch = 508.0 mm

Predicted Data for Indicated Charges of the Following Powders.
Matching Maximum Pressure: 50000 psi, or 344 MPa
or a maximum loading ratio or filling of 105 %
These calculations refer to your specified settings in QuickLOAD 'Cartridge Dimensions' window.
C A U T I O N : any load listed can result in a powder charge that falls below minimum suggested
loads or exceeds maximum suggested loads as presented in current handloading manuals. Understand
that all of the listed powders can be unsuitable for the given combination of cartridge, bullet
and gun. Actual load order can vary, depending upon lot-to-lot powder and component variations.
USE ONLY FOR COMPARISON !

76 loads produced a Loading Ratio below user-defined minimum of 80%. These powders have been skipped.
Powder type          Filling/Loading Ratio  Charge    Charge   Vel. Prop.Burnt P max  P muzz  B_Time
                                      %     Grains    Gramm   fps     %       psi     psi    ms
---------------------------------  -----------------------------------------------------------------
Nitrochemie A/S 0300               101.7     30.7     1.99    2619    98.3    50000    5932   0.988  ! Near Maximum !
SF033 fl, russ. 5,45x39            105.0     31.6     2.05    2599    99.8    47728    5735   1.009  ! Near Maximum !
Norma 200                          104.1     30.0     1.94    2572    98.9    50000    5657   1.020  ! Near Maximum !
ADI AR 2219                        103.4     31.4     2.04    2568    94.8    50000    5832   0.993  ! Near Maximum !
SNPE Vectan SP 10                  103.6     32.6     2.11    2568    93.6    50000    5895   0.999  ! Near Maximum !
PB Clermont PCL 508 (PCL223)       103.5     32.6     2.11    2566    93.6    50000    5886   1.000  ! Near Maximum !
ADI AR 2207                        101.6     28.0     1.81    2565   100.0    50000    5334   1.002  ! Near Maximum !
Hodgdon H335                       103.7     33.3     2.16    2562    93.4    50000    5897   0.998  ! Near Maximum !
IMR 4198                           105.0     27.7     1.80    2526    98.8    49419    5276   1.000  ! Near Maximum !
Vihtavuori N120                    104.1     28.0     1.81    2524   100.0    50000    4933   1.006  ! Near Maximum !
Alliant Reloder-7                   99.9     28.4     1.84    2522    98.6    50000    5337   1.017  ! Near Maximum !
Vihtavuori N125                     97.8     26.8     1.73    2515   100.0    50000    4919   1.011  ! Near Maximum !
Alliant Reloder-10x                105.0     29.2     1.89    2513    97.5    45650    5627   1.043  ! Near Maximum !
Accurate 5744                       86.2     24.3     1.57    2508    98.8    50000    5271   1.006  ! Near Maximum !
Lovex D060                          88.2     24.3     1.57    2508    98.8    50000    5271   1.006  ! Near Maximum !
Hodgdon H4227                       95.1     25.6     1.66    2501    99.9    50000    4952   1.004  ! Near Maximum !
Kazan Sunar 308W                   104.1     30.0     1.94    2501    97.5    50000    5281   1.016  ! Near Maximum !
ADI AR 2210                        103.0     31.3     2.03    2499    91.1    50000    5530   1.014  ! Near Maximum !
Accurate 1680                       91.4     28.1     1.82    2499    96.1    50000    5315   1.012  ! Near Maximum !
Accurate 2200                       99.8     29.8     1.93    2497    92.5    50000    5448   1.012  ! Near Maximum !
Lovex D073.4                       105.0     33.1     2.15    2497    90.6    48352    5591   1.028  ! Near Maximum !
Nitrochemie A/S 0200                86.1     25.3     1.64    2494   100.0    50000    4886   1.019  ! Near Maximum !
Hodgdon H110                        80.2     25.3     1.64    2490   100.0    50000    4813   1.027  ! Near Maximum !
ADI BM2                            100.5     31.5     2.04    2489    91.1    50000    5465   1.017  ! Near Maximum !
Lovex D063                          83.3     25.6     1.66    2484    98.8    50000    5035   1.007  ! Near Maximum !
IMR 4227                            93.9     25.3     1.64    2478    99.6    50000    4892   1.008  ! Near Maximum !
Lovex S053                          95.8     25.3     1.64    2477   100.0    50000    4677   1.018  ! Near Maximum !
Bofors RP1 NT ~approximation        99.9     28.5     1.85    2476    94.1    50000    5211   1.011  ! Near Maximum !
Hodgdon H4198                      100.5     27.8     1.80    2469    94.5    50000    5132   1.013  ! Near Maximum !
Accurate 2230                      105.0     33.5     2.17    2460    87.7    47184    5477   1.038  ! Near Maximum !
Accurate 2460                      105.0     33.3     2.16    2457    89.7    44768    5605   1.063  ! Near Maximum !
Winchester 748                     105.0     33.3     2.16    2454    88.0    43539    5760   1.057
Rottweil R910                       82.8     22.5     1.46    2452   100.0    50000    4227   1.015  ! Near Maximum !
Lovex D073.5                       105.0     33.2     2.15    2448    89.5    44249    5587   1.068  ! Near Maximum !
Hodgdon H322                       105.0     30.0     1.94    2447    92.6    42787    5571   1.062
Somchem S321                       105.0     33.3     2.16    2446    88.0    43600    5684   1.064
ReloadSwiss RS 30                   89.2     23.1     1.50    2442   100.0    50000    4371   1.017  ! Near Maximum !
Alliant Reloder-12                 105.0     31.5     2.04    2434    93.0    43730    5432   1.060
ReloadSwiss RS 40                  105.0     31.5     2.04    2431    87.5    45850    5430   1.055  ! Near Maximum !

When I reloaded for the 7.62x39, I got my best velocity and accuracy using Reloader 7 under a 123gr Hornady FMJ.

I can't remember the loading data, but that was the powder I used in the Russian SKS.

Accurate 2200 does well for me. I use 29 grains with a 125 bullet, but that is for Chinese-made autoloaders. In a front-locking turnbolt action there is no need to limit pressures to 50K (psi), so you can pump it up a grain or so to reach the same velocities as your "factory" loads.